1. Field of the Invention
This invention relates in general to methods and devices for cellular telephone communications, and in particular to a method and apparatus for intelligent microcell and antenna selection in cellular telephone systems.
2. Description of Related Art
Cellular telephone communications technologies has advanced rapidly in the last decade. A key advance in the development of cellular telephone communications is the advent of digital microcells, as described in U.S. Pat. Nos. 4,932,049 and 5,504,936, issued to Lee, both of which are hereby incorporated by reference herein. These patents describe the use of a plurality of contiguous cells, each cell with a master site and a plurality of zone sites defining communication zones, which are defined by a plurality of suitably positioned antenna sets within the periphery of the cell. Consequently, each zone has a transmitting and receiving means directionally configured to limit propagation of signals substantially to a zone within the boundaries of the cell.
The performance of cellular telephone systems, i.e., voice quality, coverage, and capacity, has been improved through the use of both intelligent microcells and beamswitching or beam-forming antennae defining communications zones. This may be accomplished by measuring the strength of the signal transmitted by the mobile telephone in each communication zone, and use of that measurement to determine the location of the mobile cellular telephone. Thereafter, transmissions to that mobile cellular telephone are accomplished by selecting the antennae transmitting into the same communication zone that the mobile cellular telephone is located in. By substantially limiting transmissions into this particular communication zone, interference in other zones is minimized and capacity is increased.
Another major improvement in mobile cellular telephone technology involves the application of code division multiple access (CDMA) communication technologies. CDMA is a method wherein each user's transmissions are modulated by a different pseudo-random (PN) code to distinguish the user's logical channel, instead of being assigned a different frequency channel or a different time slot. These PN codes are orthogonal or partially correlated to each other, so that it is possible to identify the user channel based on an analysis of the PN codes used in the transmission. As a result, more than one user can use the same frequency, so long as they each use different PN codes to modulate their transmissions.
CDMA is especially desirable if the communication channel is relatively noisy. This is because CDMA typically uses spread spectrum techniques that are tolerant to noise and multipath interference. As a result, CDMA allows more users to use more noisy frequency channels to make calls, thereby increasing the capacity of the cellular telephone system.
Another advantage of CDMA is that every cell uses the same set of wideband frequencies or channels. As a result, it is possible to have the closest co-channel separation, i.e., the ratio of the co-channel separation distance (D) and the cell radius (R) in a CDMA system could be equal to 2, whereas the ratio D/R for other communication methods is about 4.6.
A consequence of using the same set of wideband frequencies in every cell is that no frequency switching is required as mobile units move from cell to cell. Instead, as mobile units move from cell to cell, only the PN codes need to change. Such changes in the PN codes, instead of frequency, are referred to as "soft" hand-offs. As a result, there is an improvement in the performance of the cellular telephone system.
While intelligent microcell and adaptive antenna switching technologies have been successful, they do not take full advantage of the benefits offered by microcell cellular telephone technology. For example, the foregoing increases capacity and reduces noise and cost by modifying the transmission of signals from the cell to the mobile cellular telephone to reduce interference, but does nothing to reduce received noise from other extraneous sources that are not under the cellular telephone network's control. There is therefore a need in the art for improved methods to reduce these noise sources.
Further, intelligent microcells and beam-switching antennae have not been successful CDMA cellular telephone systems. A primary reason for this is that the PN code for each user is not detected at the RF (radio frequency) stage in order to determine the associated signal strength. As a result, the proper microcell zone or antenna beam cannot be selected by the signal strength. Thus, there is also a need in the art for microcell and antenna selection in CDMA cellular telephone systems.